What is the definition or description of: flexor muscle?

Opposes extension: Flexor muscles cause a muscle to tighten. Their function is most noticeable in large muscle groups. In your arms when you "make a muscle" you are flexing. Making a fist or curling your toes are all acts of flexion. Relaxing your limb is called extension. Throughout the day you do both. Check out anatomy pictures in a book or online.